comment:

The presence of a small amount of intracytoplasmic pigment within the splenic red pulp macrophages is a common background finding in rodents. Pigment can be hemosiderin, ceroid/lipofuscin, melanin, or test article related. It is typically found in the red pulp (
Figure 1
, arrow) but may also be present in the marginal zone and/or periarteriolar lymphatic sheaths of the white pulp (
Figure 2
, arrow), the capsule, or trabeculae. Hemosiderin pigment is the result of macrophage engulfment of red blood cells (usually effete or damaged) or free hemoglobin and may be increased in aging rodents and/or such conditions as treatment-induced hemolytic anemia or methemoglobinemia, chronic congestion, increased hematopoietic cell proliferation, and erythrophagocytosis. Hemosiderin deposits may also be observed with focal hemorrhage secondary to malignant neoplasms. The incidence of hemosiderin pigment in aging rodents is usually higher in females than in males for both rats and mice. Ceroid/lipofuscin, derived from oxidation and polymerization of unsaturated lipids, may also be present within splenic macrophages but occurs less commonly than hemosiderin. Hemosiderin can be identified with Perl’s Prussian blue special stain, whereas ceroid/lipofuscin is acid-fast positive. Melanin pigment can be seen in trabecular melanocytes or focally in the red pulp of black mice. Fontana-Masson or Schmorl’s ferric-ferricyanide reduction stain can be used to identify melanin granules. Pigment may be subclassified by type (i.e., hemosiderin, ceroid/ lipofuscin, melanin) only when confirmed by special stains.

recommendation:

Pigment in the red and/or white pulp of the spleen should be diagnosed when increased or decreased due to treatment and assigned a severity grade. A diagnostic modifier (i.e., "increased" or "decreased") should be included in the diagnosis to indicate whether the pigment is increased or decreased relative to concurrent controls. Definitive pigment identification is often difficult in histological sections, even with a battery of special stains. Therefore, a diagnosis of "pigment" (as opposed to diagnosing the type of pigment, e.g., hemosiderin or lipofuscin) is most appropriate. Morphological features of the pigment should be described in the pathology narrative. If the presence of pigment is secondary to age or not toxicologically meaningful, the pigment should not be diagnosed. The pathologist should use his or her judgment in deciding whether secondary deposits of pigment are prominent enough to warrant a separate diagnosis.